1. Field of the Invention
The present invention relates to a dielectric composition for a multilayer ceramic capacitor, a multilayer ceramic capacitor and a method for manufacturing the multilayer ceramic capacitor, and more particularly to a dielectric composition for a multilayer ceramic capacitor in which its dielectrics are formed to have a thin layer structure, a multilayer ceramic capacitor using the dielectric composition, and a method for manufacturing the multilayer ceramic capacitor.
2. Description of the Related Art
Recently, there has been an increasing trend toward high integration, miniaturization and weight reduction of components in the electronic industry.
As for a multilayer ceramic capacitor, a higher capacitance and a smaller size thereof have also been strongly pursued, along with securing an excellent heat resistance and a good reliability.
Such a multilayer ceramic capacitor is generally prepared through sintering after lamination of a dielectric slurry and paste of internal electrodes, for example, using a sheet process and a printing process.
As for internal electrodes, Ni or Ni alloys are preferred due to their lower price, although Pd or Pd alloys have been generally used.
In case of forming such internal electrodes with Ni or Ni alloys, there is a problem of oxidation of the electrodes when sintering in the atmosphere.
Thus, after a de-binder process, the inner electrodes are generally sintered at a partial pressure of oxygen lower than the equilibrium partial pressure of oxygen under the atmosphere of Ni and NiO, and the dielectric layers are re-oxidized by heat treatment thereafter (Japanese Patent Laid-open Publication No.(Hei)3-133116).
However, sintering under a reducing atmosphere causes reduction of the dielectric layers and a reduction in specific resistance.
Thus, there is proposed a reduction resistant dielectric composition that is not reduced after being sintered under a reducing atmosphere.
However, the multilayer ceramic capacitors using the reduction resistant dielectric material have problems in that the accelerated life of insulation resistance (IR) under a higher temperature and the reliability thereof are deteriorated.
Further, there is another problem of lowering of the relative dielectric constant of the dielectric layer with the passage of time, particularly remarkable under the direct current.
In order to provide a multilayer ceramic capacitor with a higher capacitance and a smaller size, it is necessary to provide a thinner dielectric layer.
In case of forming a thinner dielectric layer, an increase in the electric field strength under the direct current is involved.
Thus, the variation with the passage of time in the relative dielectric constant becomes remarkable.
As for materials for the dielectrics having the X7R characteristics regulated in EIA standard, for example, Japanese Patent Laid-open Publication No.(Sho)61-36170 discloses BaTiO3+SrTiO3+MnO-based composition.
In order to provide the X7R characteristics, the variation in capacitance should be within ±10% (at the reference temperature of 25° C.) at a temperature of −55° C.˜125° C.
However, the BaTiO3+SrTiO3+MnO-based composition exhibits a great variation in capacitance with the passage of time under the direct current electric fields. For example, in case of applying a direct current of 50 V at 40° C. for 1000 hours, the variation in capacitance approaches 10% to −30%, thereby not satisfying the X7R characteristics.
Further, in accordance with the B property (EAIJ standard) which is temperature characteristics of capacitance, the variation in capacitance is defined to be within ±10% (at a reference temperature of 20° C.) at a a of between −25° C. and 85° C.
Further, as for other dielectric ceramic composition having the reduction resistance, Japanese Patent Laid-Open Publication No.(Sho)57-71866 discloses BaTiO3+MnO+MgO-based compositions.
However, if the dielectric layer with any of the dielectric ceramic compositions mentioned above is formed to have an ultra thin layer structure having, for example, a thickness of 4 μm or less, all properties, such as temperature characteristics of capacitance, variation in capacitance with the passage of time under the direct current electric fields, accelerated life of insulation resistance, deterioration in capacitance under the direct current bias, etc., cannot be satisfied concurrently.
For example, the dielectric ceramic compositions disclosed in Japanese Patent Laid-open Publications No.(Sho)61-250905 has problems of a reduction in the accelerated life of insulation resistance or a serious deterioration in capacitance under the direct current bias.
In order to overcome the problems as disclosed above, Japanese Patent Laid-open Publication No. 2000-311828 discloses a method for manufacturing a dielectric ceramic composition and a method for manufacturing an electronic component having dielectric layers.
In accordance with Japanese Patent Laid-Open Publication No. 2000-311828, the dielectric ceramic composition comprises at least a main component represented by the formula BamTiO2+n where 0.995≦m≦1.010, 0.995≦n≦1.010 and 0.995≦Ba/Ti≦1.010 based on the molar ration, a subcomponent which is a sintering agent mainly comprising silicone oxide, and other subcomponents. The method of the invention comprises the steps of: preparing powder which is not binder-burnt out by mixing the main component and at least a part of other subcomponents except the subcomponent, preparing a binder-burnt out powder by binder-burning out the non binder-burnt out powder, and preparing a dielectric ceramic composition having a predetermined molar ratio of the respective subcomponents to the main component by at least mixing the binder-burnt out powder with the secondary subcomponent.
As for subcomponents in the above dielectric ceramic composition, the proposed is MgO, CaO, BaO, SrO and Cr2O3; (Ba, Ca)xSiO2+x(x=0.8˜1.2); V2O5, MoO3 and WO3; R(which is at least one element selected from Y, DY, Th, Gd and Ho).
The dielectric ceramic composition as discussed above has advantages of a lower variation with the passage of time and a lower reduction in capacitance under the direct current electric fields, and of a longer accelerated life of insulation resistance.
However, since the above composition must be sintered at a temperature of about 1300° C., it has a problem in that it cannot be applied to the dielectrics for a capacitor having a superior capacitance, which is formed to have the thin layer structure of a thickness of 4 μm or less.
That is, in case of using the composition as a dielectric material for a multilayer ceramic capacitor having a higher capacitance, in which Ni internal electrodes are disposed between dielectric layers, when sintering the dielectric layers and Ni internal electrodes concurrently, the Ni internal electrodes shrink earlier than the dielectric layers at a low temperature due to their lower sintering temperature. De-lamination occurs between the layers and the internal electrodes are massed into ball at their ends, thereby causing a short circuit.